Christopher (much like Randy) had always been jealous of the fact that it was Philionel, and not him, who was the Crown Prince. So naturally, when an assasination plot against Prince Phil began, everyone suspected Christopher. Therefore it came as a huge shock for everyone to learn that it was actually Alfred who was behind it all. In the novels Christopher blamed himself what happened, and wound up killing Alfred to stop him from doing anything else.
In the anime, Christopher was not the one who killed Alfred (instead it was Mazenda), although he did renounce his claim to the throne when Philionel disappeared.